Connecting mini wind turbine to charge phone

[ajob]

Hello,

I made a scale horizontal axis wind turbine that I would like to connect to my phone to charge it. I am trying to decide which motor to buy. Here is my plan and question:

Plan:
The wind turbine uses a 12 volt dc motor so that when the wind makes the motor shaft rotate there is a potential difference of 12 volts in the output. I will connect the red, positive, wire to a 7805 ic voltage regulator to bring down the voltage to 5 volts (so that my phone doesn't short) and then connect the output wires to a USB charger cable that i plug the phone into.

Question:
There are several 12 volt dc motors I could use, with RPM ratings (with no load) from 100 RPM to the many thousands of RPM. What should I use and what is the difference (electrically, obviously the mechanical difference is the RPM). My guess is it works something like this:
If you have a 12V 100RPM dc motor and you spin the shaft yourself at 50 RPM then the motor won't produce the full 12 volts (maybe around 6?), and if you spin the shaft at 100 rpm yourself then the motor will produce the full 12 volts. Whereas if you have a 12 volt 6,000 rpm dc motor then you would have to somehow manage to spin the shaft at 6,000 rpm in order for the motor to produce the full 12 volts. What is the relationship between the RPM you spin the shaft at and the voltage the motor generates--is it a linear relationship, logarithmic, etc?? Also does the 100 rpm 12V motor have a higher mechanical resistance to spinning the shaft than the 6,000 rpm 12V motor (not resistance in terms of V=IR, just resistance in terms of how hard it is to rotate the shaft at all)? Thank you for the help!

 

[Asymptotic]

How fast does the turbine shaft turn? The answer suggests what the generator (motor) speed should be.

A permanent magnet DC motor acts as a generator with a linear relationship between shaft speed and voltage output.

Also does the 100 rpm 12V motor have a higher mechanical resistance to spinning the shaft than the 6,000 rpm 12V motor (not resistance in terms of V=IR, just resistance in terms of how hard it is to rotate the shaft at all)?

Probably yes. A motor rated 100 RPM full speed likely means it it geared, and will require more torque to turn. Read through this prior discussion about the relationship between torque and power to gain a better sense of it.

 

[sophiecentaur]

It's worth finding out how fast the turbine will rotate in a wind that you are likely to encounter when not standing in a hurricane. Then see what the effect is when you apply a light braking with your hand round the shaft. That could suggest the sort of power you are likely to get out.

 

[CWatters]

Might be of interest. Uses the fan from a PC..

http://www.instructables.com/id/5-Bicycle-Cell-Phone-Charger-Wind-Turbine-with-bui/